Electronic packaging technology has undergone dramatic changes during the past decade, driven by reliability, size and price considerations. Newer packaging technologies are competing with or replacing dual in-line packages (DIP) and plastic leaded chip carriers for single and multiple chip packaging. Several of these packaging methods include flip chip, chip on board, and tape automated bonding. All three methods involve the use of liquid encapsulants to protect the chip and the electrical interconnections from physical damage and ionic contamination. The encapsulants are dispensed to form a glob over the chip and the electrical interconnections, hence the term glob-top encapsulant. Glob-top encapsulation was originally introduced for consumer packages such as video games, but the demand for miniaturized circuitry led to the use of glob-top as the preferred assembly method for many types of products including smart credit cards, microprocessor circuitry, and complex hybrids. This encapsulation technology allows the manufacturer to make relatively thin devices, and many companies produce packages with cost equal to or less than conventional plastic packages.
Typical glob-top compositions include epoxy or silicone encapsulating resins which provide protection against corrosion, vibration and mechanical stresses. These resins have achieved a high degree of popularity because the dispensing process can be easily automated, high yields are obtainable, and material costs are low. Stringent performance specifications are required for microelectronic encapsulations to insure that no physical or electrical degradation of the device occurs. The purpose of an encapsulant is to protect the chip and fragile wire bonds. An encapsulant should not create unusual stresses during thermal cycling, and it should protect the chip from mechanical shock, moisture, and various chemicals. Matching the coefficient of thermal expansion to the substrate and the chip is critical for long term dimensional stability and proper sealing of leads to prevent penetration of moisture and ionic contaminants. Encapsulants must have low moisture absorption and be of high purity, i.e., a low level of ionic contaminants such as lithium, sodium, potassium, and chlorine ions. This is very important because ionic contaminants can result in corrosion of chip metallizations under conditions of high humidity.
The application of glob top encapsulation for integrated circuit (IC) packaging protection has been limited to low end consumer electronic applications because of the inferior environmental performance of this type of package compared to molded or ceramic packages. Conventional technology uses one type of resin for the printed circuit board (typically a thermoset polyimide), a second type of resin for the die attach adhesive (typically a novalac-modified bisphenol/epichlorhydrin-based epoxy and a conductive filler), and a third type of resin for the glob top encapsulant (typically a silicone-modified epoxy resin or a silicone resin). Differences in the physical and electrical properties of the diverse materials used in the printed circuit board, the adhesive, and the encapsulant result in poor adhesion of the adhesive and encapsulant to the printed circuit board, and moisture absorption through the encapsulant into the adhesive. This creates dielectric shifts and corrosion, resulting in poor electrical performance of the package. Overmolded pin grid array and ceramic pin grid array packages are the alternatives for high performance applications, but these types of packages are more expensive than glob top. The popularity of integrated circuit chips packaged in glob-top requires a solution that will provide the environmental performance and reliability found in the more expensive packages.
Clearly a glob top package which could provide the cost and size advantages of glob top with the environmental performance advantages of ceramic or molded packages would be desirous.